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Jaws: the prequel Scientists have unveiled the impeccably preserved fossilised remains of a shark that lived 325 million years ago, complete with a series of cartilage arches that supported its gills and jaws.

Because shark skeletons are made of soft cartilage, not hard bone, finding anything more than scrappy fossilised remains of teeth and vertebrae is rare.

Finding a fossil shark in an almost three-dimensional state of preservation, boasting important skeletal structures, is exceptional.

This primitive shark, named Ozarcus mapesae, may lead scientists to rethink shark evolution, erasing the notion that these beasts of the deep have remained little changed since they first appeared at least 420 million years ago.

"These things have been very successful, among the top predators in the Earth's oceans, for over 400 million years," says palaeontologist John Maisey of the American Museum of Natural History in New York, one of the researchers.

"The best analogy I can come up with is this: It's like comparing a Model T Ford with a modern automobile. They are both recognisably the same kind of thing. But they are completely different under the hood. We found the Model T of sharks," adds Maisey, whose research was published in the journal Nature.

Those cartilage arches, for example, are quite different from those in modern sharks, the scientists write. This suggests that while their general outward appearance has stayed roughly the same, important changes have occurred over time that have helped make sharks the perfect eating machines of the sea.

Maisey says Ozarcus mapesae was small by shark standards — about 60 to 90 centimetres long, with relatively big eyes. It swam in the murky waters of a shallow inland sea teeming with life including other fish and squid-like creatures in shells.

It is not the oldest shark fossil, but it is the oldest to reveal all of the anatomy of the skeleton supporting the gill arches, a vitally important element of a fish.

Sophisticated equipment

Employing sophisticated equipment at the European Synchrotron in France, the scientists used high-resolution x-rays to obtain a detailed view of the shape and organization of the arches and associated structures.

They found that the arrangement was unlike that of modern sharks but was very much like that of bony fishes like today's tuna or swordfish. Maisey says that scientists had viewed today's sharks as very primitive creatures while seeing bony fish as more advanced.

"Modern sharks cannot be considered anymore as living fossils that did not evolve since their origin. They are actually very derived compared to early cartilaginous fishes," says American Museum of Natural History palaeontologist Alan Pradel, another of the researchers.

Sharks have undergone many other changes since the time of Ozarcus mapesae, the researchers write.

Their jaws are no longer firmly bound to the cranium, giving them greater biting freedom. Specialisations in their inner ear enable them to hear very low frequency sounds, improving their hunting ability. They are better at replacing teeth. And there have been changes in their scales, among other things.

"There's lots of ways in which modern sharks are quite highly evolved and different from the Model T," says Maisey.

The shark's genus name, Ozarcus, comes from the Ozark Mountain region where it was found. Its species name, mapesae, honours Ohio University scientist Gene Mapes, who found it.